The majority of “controlled-release” drug delivery systems operate by slowing or delaying the release of a drug post-administration. While these systems are useful for certain types of drugs (e.g., because they lead to fewer peaks and troughs in the serum profile, reduced side-effects, etc.) they are unsuitable for drugs that require more complex release profiles (e.g., release in proportion to an endogenous substance such as glucose, pulsatile release at fixed or variable time points, etc). For example, the treatment of diabetes mellitus with injectable insulin is a well-known and studied case where gradual slow release of insulin is ineffective. In fact, it is apparent that the simple replacement of the hormone is not sufficient to prevent the pathological sequelae associated with this disease. The development of these sequelae is believed to reflect an inability to provide exogenous insulin proportional to varying blood glucose concentrations experienced by the patient (i.e., a truly “controlled-release” system). As a result, there remains a need in the art for alternative controlled-release drug delivery systems and in particular systems that can be controlled post-administration. The present disclosure provides such systems.